1. Field of the Invention
This invention relates to control devices for controlling the position of a marine streamer such as military towed array or a seismic streamers.
2. Description of Ralated Art
Towed arrays or seismic streamers are generally towed behind a ship and can be thousands of metres in length. For carrying out a seismic survey, a series of streamers are towed, parallel and at a predetermined depth below the water surface. Devices known as “birds” are used to control the depth of the streamer in the water. However, in general, the birds of prior art designs are not able to correct the transverse alignment of the streamers, if, for example, a cross current affects a streamer's position. There is a desire to be able to control such transverse alignment so as to prevent entangling of adjacent streamers and to provide an optimal geometry for data gathering.
Streamers are typically deployed and retrieved by means of a winch. Another problem of many prior art designs is that the birds are too cumbersome to allow them to be permanently fixed to the streamers, the birds, necessarily having to be attached or removed from the streamers at intervals as the streamers are deployed or retrieved. Furthermore, the hydrodynamic stability of prior art designs is questionable, particularly after a period of man-handling in the field.
The problem of controlling the transverse alignment of streamers is to some extent alleviated by a proposal in EP-B-0939910 which proposes a bird having two independently controllable, planar wings configured to provide both vertical and transverse depth control. The same device however is designed with a rigid length that precludes it from stowage whilst attached to the streamer.